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Chapter 7

Chapter 7. Momentum. 7.1 Momentum. Mass is inertia and momentum is inertia in motion momentum = mass × velocity momentum = mv Measured in N·s or kg·m/s Moving object can have large momentum if it has a large mass, a high speed, or both. 7.2 Impulse Changes Momentum.

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Chapter 7

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  1. Chapter 7 Momentum

  2. 7.1 Momentum • Mass is inertia and momentum is inertia in motion • momentum = mass × velocity • momentum = mv • Measured in N·s or kg·m/s • Moving object can have large momentum if it has a large mass, a high speed, or both.

  3. 7.2 Impulse Changes Momentum • Impact is force and impulse is force × time • A force sustained for a long time produces more change in momentum than does the same force applied briefly • Impulse = Ft • Impulse = change in momentum • Therefore, Ft = Δmv • Measured in N·s

  4. Math Practice • What is the momentum of a 10-kg bowling ball rolling at 3 m/s? • If the bowling ball rolls into a pillow and stops in 0.5 seconds, calculate the average force it exerts on the pillow • What impulse occurs when an average force of 20 N is exerted on a cart for 3 s? • What change in momentum does the cart undergo?

  5. Your Turn • What is the momentum of a 20-kg carton that slides at 5 m/s across an icy surface? • The sliding carton skids onto a rough surface and stops in 2 s. Calculate the force of friction it encounters. • What impulse occurs when an average force of 10 N is exerted on a cart for 2 s? • What change in momentum does the cart undergo?

  6. Case 1: Increasing momentum • Apply a large force for the longest amount of time possible • Example: Golf club hitting the golf ball • Example: Bat hitting a baseball

  7. Case 2: Decreasing Momentum • Example : Car and haystack • Example: Landing a jump

  8. Example: “Riding with the punch” • Example: Egg catch • Same change in momentum (impulse), but if the time is large, the force is small and if the time is small, the force is large

  9. 7.3 Bouncing • Impulses are greater when an object bounces • Greater impulse is required to stop an object and then “throw it back” than to simply stop it • Example: Karate chop • Example: Pelton Wheel

  10. 7.4 Conservation of Momentum • Law of conservation of momentum: in the absence of an external force, the momentum of a system remains unchanged • Momentum is conserved • Momentum is a vector quantity • It has magnitude and direction • Example: Rifle and bullet

  11. 7.5 Collisions • Momentum is conserved in collisions • net momentum before collision = net momentum after collision • Elastic Collisions • When objects collide without being permanently deformed and without generating heat

  12. Inelastic Collisions • When colliding objects become tangled or couple together

  13. Math Practice A 4-kg blob of putty moving at 6 m/s slams into a 4-kg blob of putty at rest • Calculate the speed of the two stuck-together blobs of putty immediately after colliding. • Calculate the speed of the two blobs if the one at rest was 2 kg

  14. 7.6 Momentum Vectors • Momentum is conserved in all directions (not just in a straight line)

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